Over expression of bcl-2 is an important component in the development of B-cell lymphomas and some B-cell leukemia as well as in the emergence of cellular resistance to certain anticancer drugs. The 3'-untranslated region (3'-UTR) of bcl-2 mRNA contains several AU-rich elements (AREs) that promote mRNA destabilization. We have observed that the protein, nucleolin, binds to ARE-1 in bcl-2 mRNA, potentially protecting this mRNA from nuclease degradation. Accordingly, the central hypothesis of this proposal is that nucleolin stabilizes bcl-2 mRNA through interaction with AREs in the 3'-UTR, and that ATRA, taxol and okadiac acid destabilize bcl-2 mRNA by inducing down regulation or inactivation of nucleolin. The studies proposed in the first specific aim will evaluate the effects of bcl-2 ARE's 1-4 on bcl-2 mRNA stability in extracts of HL-60, MCF-7 and purified B cells from normal volunteers and patients with B-cell chronic lymphocytic leukemia (CLL). This will be followed by testing the effects of taxol and ATRA on bcl-2 ARE mRNA stability in cell extracts. Aim 2 is to determine whether functional nucleolin-bcl-2 mRNA interactions occur in intact cells. The ability of recombinant nucleolin to stabilize the bcl-2 mRNAs containing destabilizing AREs (Specific Aim 1) will be tested using transfection assays. We will also determine if exogenous over expression of nucleolin in parental HL-60 cells or MCF-7 cells can confer resistance to ATRA (HL-60) or taxol (HL-60 and MCF-7) concomitant with increased bcl-2 mRNA and protein. Additional experiments will reveal if knockdown of nucleolin expression with siRNAs leads to destabilization of bcl-2 mRNA. The third specific aim is to identify the RNA binding domains of nucleolin that are involved in binding to bcl-2 ARE mRNA. Finally, the information generated in Aims 1-3 will guide the development of high-throughput, robotic screening of diverse chemical libraries to identify small molecules that specifically inhibit nucleolin binding to AREs in bcl-2 mRNA.